Antibodies are proteins used by the immune system to neutralize foreign substances including bacteria, viruses, fungus, and animal dander, to name a few examples. Immunoglobulin (IgG) is the most common type of antibody found in circulation. An antibody can recognize and bind to a unique molecule of a specific foreign substance, called an antigen. Depending on the antigen, this binding may directly neutralize the foreign substance (for example, by impeding a biological process essential for its invasion and survival) or may tag the antigen so that other parts of the immune system destroy the foreign substance.
Antibodies are present in a biological fluid called serum that is typically obtained from blood. In blood, plasma is the liquid component that holds the blood cells in whole blood in suspension, and the serum is the component of plasma that is devoid of clotting factors. Serum includes other proteins not used in blood clotting, electrolytes, antibodies, antigens, hormones, and any exogenous substances (e.g., drugs and microorganisms). To obtain serum samples, blood is typically allowed to clot, and centrifuged to remove cellular components, yielding serum.
Seroprotection, the protection obtained from vaccination, can be estimated by analyzing samples of patient serum to determine whether protective levels of antibodies, such as immunoglobulins, are present in the serum. For many vaccines the amount of antibodies induced has a positive correlation with the likelihood of clinical protection from disease. Accordingly, mean antibody level is often used as a measure of vaccine efficacy, and seroprotection can be considered as an identified threshold level of antibody level that is correlated with protection from disease.